This invention relates to a marine riser system having flexible flowlines. In particular, it relates to apparatus for connecting a surface facility to a subsea wellhead or gathering system.
In the production of fluid hydrocarbons from deepwater marine oil and gas deposits, a fluid communication system from the marine bottom to the surface after production is required. Such a system, commonly called a production riser, usually includes multiple conduits through which various produced fluids are transported to and from the surface, including oil and gas production lines, service and hydraulic control lines.
In many offshore production areas, a floating facility can be used as a production and/or storage platform. Since the facility is exposed to surface and sub-surface conditions, it undergoes a variety of movements. In such a zone of turbulence, heave, roll, pitch, drift, etc., may be caused by surface and near surface conditions. In order for a production riser system to function adequately with such a facility, it must be sufficiently compliant to compensate for such movements over long periods of operation without failure.
Such a marine riser is disclosed in U.S. Pat. No. 4,182,584. This compliant riser system includes (1) a lower section which extends from the marine bottom to a fixed position just below the zone of turbulence that exists near the surface of the water, and (2) a flexible section which is comprised of flexible flowlines that extend from the top of the rigid section, through the turbulent zone, to a floating vessel on the surface. A submerged buoy is attached to the top of the rigid section to maintain the rigid section in a substantially vertical position within the water. With riser systems of this type difficulties often arise in installing and maintaining the flexible conduits. Often the flexible flowline is attached to a rigid section such that the end portion adjacent the fixed or rigid portion is not attached at a normal catenary departure angle. This can result in localized stresses, causing undue wear in the flexible flowline at its terminal hardware. If a natural catenary shape is assumed by the flowline, it approaches the fixed position section pointed upwardly, nearly vertical at its point of suspension.
In a compliant riser system for deep oceanic location, a need exists for linking the surface facility to the submerged lower riser section in a manner which provides (1) lateral excursion and rotational weathervaning of a floating vessel, (2) vertical compliance for waves and tidal conditions, and (3) disconnect and repair facilities. Due to the significant weight and pressure conditions of certain flowlines, especially large petroleum-carrying conduits, each flexible flowline member can advantageously be supported in a catenary configuration between a fixed-position buoy and the surface facility. While certain advantages adhere to multiple flexible conduits of equal length, the severe environmental and operational conditions can cause tangling or chafing of the catenary flowlines, hydraulic lines, etc.
Various attempts have been made to overcome these problems, e.g., retainers to spread and hold apart the individual flexible members; however, twisting and unequal connection stresses have caused significant problems in maintaining a reliable system.
It is an object of the present invention to provide an effective compliant riser system having a flexible flowline bundle. A particular achievement is the provision of a multi-conduit catenary bundle which can be connected compactly at one end to a rotary member on the marine surface and at the opposite end to corresponding riser conduits at the riser buoy section.